Magnetic resonance scanner with electromagnetic position and orientation tracking device

ABSTRACT

A system for combining electromagnetic position and orientation tracking with magnetic resonance scanner is provided. One embodiment includes a magnetic resonance scanner defining a reference coordinate system for scanning a target. Coupled to the magnetic resonance scanner is a magnetic field source which produces a magnetic field. The magnetic field is sensed by a magnetic field sensor which produces a signal proportional to the magnetic field. The magnetic field sensor has a location relative to the reference coordinate system. The location of the magnetic field sensor relative to the reference coordinate system of the magnetic resonance scanner is determined by a location tracking device using at least a line segment model of the magnetic field source and the signal from the magnetic field sensor.

PRIORITY

The following patent application claims priority and is a continuationpatent application of U.S. patent application Ser. No. 10/390,432 thatwas filed on Mar. 17, 2003 which itself is a continuation of U.S. patentapplication Ser. No. 09/470,166, that was filed on Dec. 22, 1999 andissued as U.S. Pat. No. 6,534,982 that claims priority from U.S.provisional patent application Ser. No. 60/113,782, filed on Dec. 23,1998, entitled “MAGNETIC RESONANCE SCANNER WITH ELECTROMAGNETIC POSITIONAND ORIENTATION TRACKING DEVICE. All of the applications areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention generally relates to magnetic resonance devicesand position and orientation tracking and, more specifically, to thecombination of a magnetic resonance scanner with an electromagneticposition and orientation tracking device.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,737,794 presents a method and apparatus for determiningthe position of a remote object and its orientation relative to areference object by measuring the electromagnetic couplings betweenfield generating and sensing elements embedded in reference and remoteobjects. The apparatus measures the couplings between the fields of thereference and remote object with the generation of a plurality ofalternating current (AC) signals applied to the field generatingelements and with the measurements of the coupled signals of the sensingelements. The method utilizes a model having an approximation of thedipole magnetic field of the generating source, based on Legendrepolynomials, and computes the coupling between the generating andsensing element starting from an estimated value for the remote objectlocation. An error is computed between the measured couplings and theone computed from the estimated location. The error is then used inconjunction with the estimated location until there is a convergencewhich produces a location representative of the actual position of theremote object. When the physical dimensions of the generating andsensing elements are comparable to the separation between them, themodel fails. The model also has no means to compensate for fielddistorting elements, for example, the presence of conductive orferro-magnetic material. U.S. Pat. No. 4,849,692 further extends themethod to utilize pulsed direct current (DC) magnetic fields in place ofAC fields, but has the same limitations.

U.S. Pat. Nos. 5,558,091 and 5,600,330 describes an apparatus whichutilizes a triad of Helmholtz coil pairs for the generation of magneticfields. The method used by the apparatus for calculating the positionand orientation of a group of magnetic field sensors uses linear orquasi-linear approximation of such Helmholtz source coils. Because themagnetic field sensor must be within the quasi-linear section of themagnetic field of the Helmholtz coils, large coils are required forlocation tracking over the required area, therefore the presence of themagnetic resonance scanner will interfere with the operation of thelocation tracking device. As a result, in prior art optical methods arepreferred.

U.S. Pat. Nos. 5,617,857, 5,622,170 and 5,828,770 describes the use ofoptical tracking devices for object position and orientation tracking.These optical devices require a line of sight between the optical cameraand the position-sensing device, which limits the use of these devices.

U.S. Pat. Nos. 5,307,808, 5,353,795 and 5,715,822 each demonstratemethods for calculating the position and orientation of an RF coil,where the position of a small RF coil is computed from the magneticresonance signal emanated from a target surrounding the coil itself inthe presence of spatially encoded magnetic field gradients. Theapparatus uses the frequency of the magnetic resonance signal todetermine the RF coil location for a spatial dimension. In thisapparatus, a magnetic resonance scanner generates a quasi-linearmagnetic field gradient via the gradient coils of the magnetic resonancescanner. Because the frequency of the magnetic resonance of the materialsurrounding the RF coil is proportional to the strength of the magneticfield, the distance of the RF coil from the center of the gradient coilof the magnetic resonance scanner is assumed to be proportional,providing the location of the RF coil. The model for the magnetic fieldsource is based upon a linear approximation and does not account formagnetic field gradient deviations which occur in magnetic resonancescanners, which results in inaccuracy. Additionally, when there is nomaterial surrounding the RF coil, the device fails, since either a weakmagnetic resonance signal or no magnetic resonance signal is produced.As a result, the use of such a device is limited by these constraints.

SUMMARY OF THE INVENTION

Systems and methods for determining the position and orientation of anobject with an attached sensor in a magnetic resonance scanner aredisclosed. In general magnetic resonance scanners acquire data about atarget which reside within a homogenous region of a static magneticfield of the magnetic resonance scanner. In one embodiment of theinvention, a magnetic resonance scanner is used in conjunction with anelectromagnetic position and orientation tracking device to calculatethe position and orientation of the object within the magnetic resonancescanner and may be used to track the object and the target relative to areference coordinate frame. In this embodiment, magnetic fields are usedfor location tracking of an object in order to avoid line of sightlimitations.

A magnetic field is generated by a magnetic field source by applyingelectric current to a conductive wire loop of the magnetic field source.The generated magnetic field is sensed by a magnetic field sensor,allowing the tracking of the location of the object throughout the fieldof the magnetic field source. At the magnetic field sensor, an electricsignal is measured which is proportional to the magnetic field. Usingthe measured value of the electric signal from the magnetic fieldsensor, a measured magnetic field may be calculated by a model ofmagnetic field sensing. Using a model of the magnetic field generationof the magnetic field source, at an estimated location given in thereference coordinate frame, an estimated magnetic field can becalculated. Comparing the estimated magnetic field to the measuredmagnetic field, an error can be computed. The estimated location for themagnetic field sensor is then changed and the steps repeated until theerror between the estimated magnetic field and the measured magneticfield falls below an acceptable level. When the error falls below theacceptable level, the location of the sensor based on the estimatedlocation provides a representation of the actual location.

The magnetic field generation model used in this system includes a linesegment approximation of the shape of the magnetic field source andadditional line segments accounting for field distorting components.This model compensates for distortions in the estimated magnetic fieldat the sensor by accounting for the magnetic field generated by thegradient coils of the magnetic resonance scanner and for the fieldsgenerated by any surrounding conductive and ferro-magnetic materials.The location and direction of the line segments is determined throughtest measurements of the currents in the gradient coils and theresulting magnetic field. The Biot-Savart Law is used in the magneticfield generation model to calculate the magnetic field generated byevery line segment.

For the magnetic field sensing model, Faraday's Induction Law is used tocalculate the electric signal induced into a solenoid type coil. Modelsfor other type of magnetic field sensors can be used in place ofFaraday's Induction Law for solenoid type coils where the magnetic fieldsensing model is derived based upon the physical law of operation forthe sensor.

In the preferred embodiment, an autonomous magnetic field source ismechanically coupled to the magnetic resonance scanner. In yet anotherembodiment the magnetic field gradient generating coils of the magneticresonance scanner generate the magnetic fields for location tracking.

When an object is attached to the magnetic field sensor, such as apointer, the location of the pointer which is known by the locationtracking device in the reference coordinate frame of the magneticresonance scanner can be represented in combination with the acquireddata from the magnetic resonance scanner. In another embodiment, thedata acquisition area may be controlled by the pointer. If a single dataacquisition is controlled by the pointer and does not cover the entiretarget, the acquired data can be combined with a previously acquireddata of the remainder of the target to give a more completerepresentation of the target.

In another embodiment in which there is a sensor attached to the target,the location of the target is used to control the location of the imageacquisition of a magnetic resonance scanner, therefore the magneticresonance scanner can compensate for the movement of the target form apredetermined starting point. In yet another embodiment, the dataacquired from one data acquisition may be combined with data fromanother acquisition based on the information from the sensor trackingthe target's motion. In another embodiment, a magnetic field sensor isattached to the target in addition to the sensor attached to the pointerand therefore movement of the target and the location of the pointerrelative to the magnetic resonance scanner can be accounted for whencombining currently acquired data with a previously acquired data set.Additionally the pointer can be represented in combination with theacquired data.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description taken with theaccompanying drawings:

FIG. 1A. depicts a system for magnetic resonance imaging of a targetwith location tracking of an object.

FIG. 1B is a block diagram showing the data and signal connectionsbetween the magnetic resonance scanner and the electromagnetic positionand orientation tracking device.

FIG. 2A. depicts an electromagnetic position and orientation trackingsystem having a magnetic field source, a magnetic field sensor and aprocessor for position and orientation tracking.

FIG. 2B is a functional block diagram showing the functions which areperformed within the position and orientation tracking processor and thedata acquisition processor of FIG. 2A.

FIGS. 3A-3H show examples of the shape of a coil and the line segmentrepresentation approximating a finite-element representation of the coilin for modeling the magnetic field source.

FIGS. 4A-D show examples of magnetic field sensing coils.

FIG. 5A shows a prior art system which includes only a magneticresonance scanner in which a representation of a target is built up froma single data acquisition using the magnetic resonance scanner.

FIGS. 5B-D show magnetic resonance image displays showing examples ofcombining new data acquisitions with previous data acquisitions.

FIGS. 6A-D show magnetic resonance image displays showing examples ofdata acquisitions with motion compensation for target movement.

FIG. 7A depicts a magnet assembly for use with a combined magneticresonance scanner and an electromagnetic position and orientationtracking device.

FIG. 7B is a block diagram showing the data and signal connectionsbetween the magnetic resonance scanner and the electromagnetic positionand orientation tracking device.

FIG. 8A depicts the generation of a static magnetic field for use in amagnetic resonance scanner in one embodiment of the invention.

FIG. 8B shows a graph of the magnetic field intensities of the first andsecond static magnetic field generating coils along the Z axis of FIG.8A along with the resulting magnetic field intensity with thehomogeneous region being denoted by a substantially constant intensityregion.

FIGS. 9A-C describes the generation of the magnetic field gradients usedfor spatial encoding of nuclear spins present at the homogenous regionof the static magnetic field.

FIGS. 10A and 10B show a Radio Frequency magnet assembly for use in amagnet resonance scanner for the excitation of nuclear spins and for thedetection of a resulting signal.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The terms “location” and “position and orientation” will be usedinterchangeably in the following detailed description and appendedclaims and shall mean the position of an object, the orientation of anobject or both the position and the orientation of an object where oneor more degrees of freedom of the object, up to six degrees of freedomin three dimensional space is determined relative to a referencecoordinate frame.

The term “coil” in the following description and accompanying claimsshall mean a winding or loop of conductive material such as a coppermagnet wire through which an electric current can flow. The term “loop”is used in an electrical engineering context referring to a completecircuit for current and does not imply that the shape is necessarilycircular.

The present invention as embodied in FIG. 1A relates to tracking anobject's location relative to a magnetic resonance scanner utilizingelectromagnetic fields. Using electromagnetic fields which areindependent of magnetic resonance, allows the object being tracked to beeither internal or external to the target. In this embodiment theelectromagnetic fields are compatible with the magnetic resonance. Theterm compatible is used herein to mean that the normal operation of themagnetic resonance scanner is maintained with acceptable levels ofartifacts.

A magnetic resonance scanner, in the preferred embodiment of theinvention, is capable acquiring data for a visual representation of thespatial distributions of the nuclear spins of a target. The magneticresonance scanner runs a magnetic resonance sequence which generatesspatial field gradients and corresponding radio frequency signals. Anexample of a magnetic resonance sequence is a coordinated sequence ofevents which include superimposing magnetic field gradients over thestatic magnetic field, generating radio frequency magnetic fields,detection of radio frequency magnetic fields and acquiring the magneticresonance signal. A visual image corresponding to the induced magneticresonance signal may be produced which represents the target. Furtherdescription of the physics of a magnetic resonance is provided inPrinciples of Magnetic Resonance, C. P. Slichter, Springler-Verlag, NY,and Foundations of Medical Imaging by Zang-Hee Cho et. al., Wiley & SonsN.Y., section III, chapters 9-13, page 236-454.

Magnetic resonance scanners are generally configured to a coordinatesystem to describe human anatomy wherein the coordinate system may beoriented about the potential data acquisition region of a target. In theembodiment of this invention shown in FIG. 1A a reference coordinatesystem is established on the coordinate system of the magnetic resonancescanner. The reference coordinate system may follow the convention ofgeneral practice as shown by the axes designated X,Y, and Z. It shouldbe understood by one of ordinary skill in the art that such a referencecoordinate system may be repositioned or reoriented relative to thecoordinate system of the magnetic resonance scanner or to any object forwhich the object's location is established.

FIG. 1A. depicts a system for magnetic resonance imaging of a targetwith location tracking of an object. The system includes a magneticresonance scanner with a coupled electromagnetic position andorientation tracking system having a magnetic field source 110 and amagnetic field sensor 120 coupled to a magnet assembly 105 of a magneticresonance scanner. The magnetic resonance scanner in its magnet assembly105 includes a static magnetic field generating assembly 130, a magneticfield gradient coil assembly (gradient coils) 140, which is usually aset of three coils, and Radio Frequency (RF) coil assembly 150. Theposition and orientation tracking device includes a magnetic fieldsource 110, secured to the magnet assembly of the magnetic resonancescanner and at least one magnetic field sensor 120 for tracking anobject's location, such as a pointer 160. The magnetic resonance scanneralso includes a processor 190 (shown in FIG. 1B) for data acquisition ofthe magnetic resonance spectroscopy or imaging sequence and may includean additional processor for handling user input and output and dataprocessing and task coordination 195 (shown in FIG. 1B).

FIG. 1B is a block diagram showing the data and signal connectionsbetween the magnetic resonance scanner and the electromagnetic positionand orientation tracking device. In a magnetic resonance scanner, thepresence of the static magnetic field creates a magnetization of thenuclear spins of the target. The processor for magnetic resonance dataacquisition 190 supervises the static magnetic field generating assembly130 through built in sensors. The processor 190 is also responsible fortransmitting radio frequency signals to excite the nuclear spins of thetarget and to receive the radio frequency signals in response of thetransmission via the radio frequency coil assembly 150. To provide threedimensional spatial encoding of the nuclear spins present in the target,the processor 190 also generates time and spatial variation in thestatic magnetic field through magnetic field gradient generating coils140.

For example, the gradient coil 140 of the magnetic resonance scannermagnet assembly 105 generates a linear magnetic field gradient acrossthe target 180 encoding the nuclear spins in the target 180 according toa magnetic resonance sequence, such that, the frequency of the magneticresonance is linearly dependent on the position of the nuclear spinsrelative to the center of the gradient coil 140. Then a Fouriertransform is performed in processor 190 to retrieve the positiondependent spin density data from the acquired radio frequency signal andthe resulting data is made available to the system processor 195. Thesystem processor 195 is responsible for accepting the resulting datafrom processor 190, user input 196, for determining parameters of thedata acquisition, and for presenting the resulting data to the user 197.The processor 195 may combine data from previous data acquisitionsarchived in a data archive 198 to help the user control the dataacquisition and analyze the data presented.

The invention in the current embodiment also includes a locationtracking device. The processor of the location tracking device 192 isresponsible for generating a spatially unique magnetic field via amagnetic field source 110 and sensing the magnetic field via magneticfield sensor 120.

For example, the gradient coil 140 of the magnetic resonance scannermagnet assembly 105 (shown in FIG. 1A) generates a linear magnetic fieldgradient across the target 180 encoding the nuclear spins in the target180 according to a magnetic resonance sequence, such that, the frequencyof the magnetic resonance is linearly dependent on the position of thenuclear spins relative to the center of the gradient coil 140. Then aFourier transform is performed in processor 190 to retrieve the positiondependent spin density data from the acquired radio frequency signal andthe resulting data is made available to the system processor 195. Thesystem processor 195 is responsible for accepting the resulting datafrom processor 190, user input 196, for determining parameters of thedata acquisition, and for presenting the resulting data to the useroutput 197. The processor 195 may combine data from previous dataacquisitions archived in a data archive 198 to help the user control thedata acquisition and analyze the data presented.

At the end of the sequence, the magnetic resonance data acquisitionprocessor 190 and the processor for position and orientation tracking192 will provide new data to the system processor 195. The systemprocessor combines this data with previously acquired data from the dataarchive 198, to present to the user at the user output 197 a combinedview of the target, at the present and past times, and the location ofthe object 160, having an attached magnetic field sensor 120. The systemprocessor 195 saves this data in the data archive 198, as explainedbelow with respect to FIG. 5A-5D. The system processor 195 can alsoinstruct the processor for the magnetic resonance data acquisition 190to modify the magnetic resonance sequence to compensate for the motionof the target 180, or by itself, can make the necessary compensations,while combining data sets of the target while it has moved as explainedbelow with respect to FIGS. 6A-6D.

In the foregoing description, three processors were described, a systemprocessor, a magnetic resonance data acquisition processor and aposition and orientation tracking device processor. It should beunderstood by one of ordinary skill in the art that the functionality ofthe described processors may be combined into a single processor locatedin the magnetic resonance scanner or in the position and orientationtracking device or the functionality may be separated into additionalprocessors in either the magnetic resonance scanner or the position andorientation tracking device. At a minimum, the system including theposition and orientation tracking device and the magnetic resonancescanner must have a single processor. The number of processors withinthe system should in no way be seen as a limitation.

FIG. 2A. depicts an electromagnetic position and orientation trackingsystem 200 having a magnetic field source 210, a magnetic field sensor220 and a processor for position and orientation tracking 292. Themagnetic field source also includes elements which are not directlydriven by the processor 292, but are magnetically coupled and theseelements can distort the generated magnetic field. In the preferredembodiment as shown in FIG. 2A, the magnetic field source is atetrahedron. In such a three dimensional shape, the coil's axes are notorthogonal, and the generated magnetic field of each coil is mutuallydistinguishable and provides the means to obtain the maximum number ofdegrees of freedom for the magnetic field sensor. In the preferredembodiment, the tetrahedron is composed of 3 triangular shaped coils. Atriangular shaped coil provides the minimum number of linear elementsthat can be used to create a closed loop coil. Additionally, thetetrahedron shape is also preferred as it eliminates ambiguity in thedetermination of the position and orientation of the magnetic fieldsensor in certain situations. For example, a coil may be reoriented to anew location in such a manner that the polarity of the generatedmagnetic field changes to the opposing polarity while the intensity mapof the generated magnetic field remains the same. Applying the samemovement to the entire magnetic field source assembly, in which thepolarity of the generated magnetic field changes to the opposingpolarity for one coil while its magnetic field intensity map remains thesame, changes the magnetic field intensity map of the other two coilsfrom their original magnetic field intensity map. This provides a meansfor distinguishing between the two orientations of the magnetic fieldassembly. If symmetry existed between the normal and reverse intensitymap due to the shape of the magnetic field source assembly, reversingthe direction of the magnetic field source assembly and the magneticfield sensor would create ambiguity as to the position and orientationof the magnetic field sensor. Further, the tetrahedron shape allows fora fourth coil which can provide redundancy to the location trackingdevice creating further asymmetry to avoid ambiguity.

FIG. 2B is a functional block diagram showing the functions which areperformed within the position and orientation tracking processor and thedata acquisition processor of FIG. 2A. The process starts with thegeneration of an electric current by an electric current generator 215through the magnetic field source 210. The magnetic field sensor 220senses the magnetic field 205 of the magnetic field source 210outputting an electric signal in response and proportional to themagnetic field 205 of the source 210. The current in the magnetic fieldsource 210 and the signal which is measured by the magnetic field sensor220 is converted from an analog representation to a digitalrepresentation to enable further processing to be performed in thedigital domain. The electric current of the magnetic field source 210 isconverted into a digital format by the magnetic field signal sourceconverter 232. The signal output of the magnetic field sensor 220 isdigitized by a magnetic field sensor signal converter 234. A magneticfield source signal correlator 236 then can calculate theauto-correlation function of the measured current of the magnetic fieldsource. A magnetic field sensor signal correlator 238 calculates across-correlation function between the measured current of the magneticfield source and the measured signal of the magnetic field sensor. Thisprocess rejects the time variation of the signals and extracts from themeasured signal of the magnetic field sensor the position andorientation dependency of the magnetic field sensor.

To perform various signal processing as previously discussed in ananalog or digital domain, such as amplification, filtering, analog todigital conversion, digital to analog conversion, computing an integralof a function, computing cross-correlation between two functions,auto-correlation of a function, and finding minima of a function,standard engineering practices are used, such practices are described inThe Electrical Engineering Handbook, Editor-in-Chief R. C. Dorf, CRCPress, FL, Signals and Systems by A. V. Oppenheim and A. Willsky with I.T Young, Prentice-Hall, Digital Signal Processing by A. V Oppenheim andR. W Schafer, Prentice-Hall, Discrete-Time Signal Processing, by A. V.Oppenheim and R. W. Schafer, Prentice-Hall, NJ. Numerical Recipes in C,W. H. Press, S. A. Teukolsky, W. T. Vetterling, B. P. Flannery,Cambridge University Press, MA, and Handbook for Digital SignalProcessing, Edited by S. K. Mitra and J. F. Kaiser, John Wiley an Sons,NY.

The magnetic field generation model 254 as described below with respectto FIGS. 3A-H is used to calculate the estimated magnetic field matrix256 based on a location estimate 258, input parameters 252 of themagnetic field generation model, and the output of the magnetic fieldsource signal correlator 236. The measured magnetic field matrix 257 iscalculated by the magnetic field sensing model 255 as described withrespect to FIGS. 4A-D below based on the input parameters of the modelof magnetic field sensing 253 and the output of the magnetic fieldsensor signal correlator 238. The measured and the estimated magneticfield matrixes 256, 257 are compared by a field matrix comparator 251which produces an error value for the location estimator 258. The modelfor the magnetic field is initialized with an estimated location for themagnetic field source. This value may be manually set or may default toa position within the magnetic resonance scanner. For instance, thecenter of the magnetic resonance scanner could be the estimated locationof the magnetic field sensor. It should be understood by one skilled inthe art that the magnetic resonance scanner has an associated referencecoordinate system wherein the origin of such a system may be configuredto a point within or exterior to the magnetic resonance scanner. Forexample the origin of the system may be at the physical center of themagnetic resonance scanner.

The location estimator 258 generates a new estimate of the location ofthe magnetic field sensor until the error generated by the magneticfield matrix comparator 251 is below a given threshold. When thisthreshold is reached, the estimated location of the magnetic fieldsensor is an acceptable approximation of the real location of themagnetic field sensor 220. The location estimator determines thelocation through a minima finding algorithm. Once the location isdetermined it is then output. From output 259 the position andorientation of the magnetic field sensor may be displayed on a monitor(not shown), printed or stored in an associated memory location.

To determine the six unknowns for a six degree of freedom position andorientation tracking device at least six independent magnetic fieldmeasurements are required. Utilizing three gradient coils of themagnetic resonance scanner and at least two, but preferably threesolenoid coils, nine independent measurements are possible. A Cartesiandecomposition of the measurements of the magnetic field results in themeasured magnetic field matrix 256. Accordingly, the estimated magneticfield matrix 257 will be equal in size to the measured magnetic fieldmatrix 256. Creation of the measured magnetic field matrix 256 isdescribed below with respect to FIGS. 3A-H and the creation of theestimated magnetic field matrix is described below with respect to FIGS.4A-D.

FIG. 3A shows an example of the shape of a coil which is a Golay coiland FIG. 3B shows the line segment representation approximating afinite-element representation of the Golay coil, included in one modelof the magnetic field source. The combination of the line segmentrepresentation in conjunction with the means for calculating themagnetic field at any location provides a sufficient description of themodel of the magnetic field generation. The means for calculating themagnetic field is provided by a computer processor operating withcomputer code to calculate the Biot-Savart Law. The Biot-Savart Lawexpresses the relationship between the magnetic flux density generatedby one piece of current carrying wire segment at a given location. For awire element of d{overscore (l)} the magnetic flux density d{overscore(B)} is:${d\quad\overset{\_}{B}} = {\frac{\mu_{0}I}{4\pi}\frac{d\quad\overset{\_}{l} \times \overset{\_}{r}}{r^{3}}}$

Where μ₀ is the magnetic permeability of free space, I is the intensityof the electric current in the wire, {overscore (r)} is the vectorpointing from the wire element to the location of the magnetic fieldbeing calculated. A more complete explanation of the Biot-Savart law isprovided in Classical Electrodynamics by J. D. Jackson, Wiley & SonsN.Y., chapter 5, 168-208 that is incorporated by reference in itsentirety. To compute the estimated magnetic field at an estimatedlocation of the magnetic field sensor, the sum of d{overscore (B)} foreach wire element over the entire length of the magnetic field model iscalculated, utilizing the measured value of the current flowing into thegradient coils. Using the model as described above for each element ofthe magnetic field source given the current in the element, theestimated magnetic field at the estimated location of each element ofthe magnetic field sensor is calculated. The resulting values for theestimated magnetic field forms a matrix better described as theestimated magnetic field matrix.

FIG. 3C shows a ring and FIG. 3D shows the line segment representationof the ring of FIG. 3C. The ring of FIG. 3C or any other shaped coil isadded to the model of the magnetic field generation to describe fielddistortions produced by conductive and ferro-magnetic material presentin the magnetic resonance scanner other than the gradient coils. Thelocation and direction of all the line segments present in the model aredetermined through test measurements of the currents in the gradientcoils and the resulting magnetic fields.

FIGS. 3E-H show structures with triangular shapes which can be used torepresent the magnetic field source. Such structures may be added to analready existing magnetic resonance scanner. In the preferredembodiment, the gradient coils are used as the magnetic field source. Asexplained above, a triangular coil provides the simplest structure whichhas a match between an actual coil shape and a line segmentrepresentation. It should be clear to one of ordinary skill in the artthat other line segment shapes may be used with equal accuracy. In anembodiment which uses such structures, the coils are rigidly coupled tothe magnetic resonance scanner and are preferably built into themagnetic resonance scanner's magnet assembly. By providing externalmagnetic field generating elements a complete position and orientationtracking system may be constructed independent of a magnetic resonancescanner through the above described techniques.

FIGS. 4A-D show examples of magnetic field sensing coils. FIG. 4A showsa single solenoid coil, FIG. 4B shows three ring coils with their axesorthogonal to one another and FIG. 4C is three adjacent rectangularcoils with their axes orthogonal to one another. FIG. 4D shows threeco-located coils with their axes orthogonal to one another. The solenoidcoil is preferred when only five degrees of freedom of an object issought. The three ring shaped coils or the three rectangular shapedcoils with their axes mutually distinguishable in combination with amagnetic field source having at least two coils with their axes mutuallydistinguishable provides sufficient information to determine six degreesof freedom of an object. The shape of FIG. 4D provides a compact volumefor three coils and additionally provides for easy manufacture. Themagnetic field sensing coils of FIGS. 4A-D are shown for exemplarypurposes only and other shapes or types of magnetic field sensors may beused with embodiments of the invention as described, for example, Halleffect devices, flux gate devices, and Kerr-effect devices. For eachmagnetic field sensor, a magnetic field sensing model is necessary whichdescribes the relationship between the output signal and the magneticflux density.

For a magnetic field sensor, which is constructed from a single solenoidtype coil, the magnetic field sensing model is computed using theFaraday's Induction Law assuming that the magnetic field is constantover the surface area of the coil. The Faraday Law defines the magneticflux linking φ of a coil as:$\phi = {{\int_{A}^{\quad}{{\overset{\_}{B} \cdot \overset{\_}{n}}\quad{\mathbb{d}a}}} = {{\overset{\_}{B} \cdot \overset{\_}{n}}\quad A}}$

The electromotive force E, in the coil is:$E = {{- L}\frac{\mathbb{d}\phi}{\mathbb{d}t}}$

Where {overscore (n)} is the normal of the coil having a surface area A,and inductance L. As described in Classical Electrodynamics by J. D.Jackson, Wiley & Sons N.Y., chapter 6, pages 209-223, which isincorporated by reference in its entirety herein.

A signal representative of the electromotive force which is created inresponse to the magnetic field for each magnetic field source element ismeasured for each element of the magnetic field sensor and from theelectromagnetic field sensing model described above a measured magneticfield matrix may be created.

FIG. 5A shows a prior art system which includes only a magneticresonance scanner in which a representation of a target is built up froma single data acquisition using the magnetic resonance scanner. In FIG.5A the target is smaller than the area of the data acquisition area ofthe magnetic resonance scanner and, as such, the a representation of theentire target may be produced on the magnetic resonance data display.FIG. 5B shows one embodiment of the present invention in which amagnetic resonance scanner and a position and orientation trackingdevice are coupled together. In this embodiment, an object, such as apointer, is attached to the magnetic field sensor and the location ofthe pointer which is known by the location tracking device in thereference coordinate frame of the magnetic resonance scanner can berepresented in combination with the acquired data as shown by thecross-hair. FIG. 5C shows the location of the pointer determining thelocation of the data acquisition of the magnetic resonance scanner. Thepointer's location which is known by the location tracking device isassociated with a fixed area acquisition region. The selectedacquisition area of the target may be displayed on the magneticresonance data display. FIG. 5D shows the pointer with the attachedmagnetic field sensor. The pointer again provides a data acquisitionarea for the magnetic resonance scanner which does not cover the entiretarget. In FIG. 5D, the pointer is used to direct the magnetic resonancescanner to obtain data from an area of the target which has changedsince a previous magnetic resonance scan of the target has occurred.Since the position and orientation tracking device knows the location ofthe pointer and therefore the data acquisition area relative to thereference coordinate system of the magnetic resonance scanner, thesystem can combine the data acquisition of the changed area of thetarget with a previously acquired data set of the target to give a morecomplete representation of the target. If in the previously acquireddata set, the data set comprised a representation of the target, theposition and orientation tracking device could substitute the newlyacquired data set in the proper location in the previously acquired dataset based on the location information.

In another embodiment in which there is a sensor attached to the target,as shown in FIG. 6A-B, the location of the target is used to control thelocation of the image acquisition area of the magnetic resonance scannerand therefore the magnetic resonance scanner can compensate for themovement of the target as shown in FIG. 6B from a predetermined startingpoint as shown in FIG. 6A. The location tracking device allows thepredetermined starting point to become a point of reference andtherefore any deviation from that point by the target can be accountedfor on the magnetic resonance data display. In yet another embodiment,the data acquired from one data acquisition may be combined with datafrom another acquisition based on the information from the sensortracking the target's motion as shown in FIG. 6C. The movement of thetarget relative to a predetermined starting position is tracked and thendata sets can be overlaid in the proper position accounting for anymovement by the target. In another embodiment, a magnetic field sensoris attached to the target in addition to the sensor attached to apointer as shown in FIG. 6D. Therefore, movement of the target and thelocation of the pointer relative to the magnetic resonance scanner canbe accounted for when combining currently acquired data with apreviously acquired data set. In this embodiment, the pointer points tothe data acquisition region which is less than the entire target. Sinceboth the target and the pointer have sensors attached to them, which arebeing tracked relative to the magnetic resonance scanner, the data setfrom the scanner that is acquired can be related to another data set inwhich the target was not in the same position. This allows the data setsto be overlaid and positioned properly so that a combined representationof the target may occur. The pointer can also be represented along withthe acquired data as shown by the crosshair. In FIG. 6D the currentlyacquired data set representing the target has a change as shown in theupper left corner of the target, which is acquired in the shadedacquisition area of the central diagram. In the diagram to the far rightthe acquired data set with the change is overlaid with a previouslyacquired data set, so that the data display accounts for the change tothe target.

FIG. 7A depicts a magnet assembly 705 for use with a combined magneticresonance scanner and an electromagnetic position and orientationtracking device. The magnetic resonance scanner in its magnet assembly705 includes a static magnetic field generating assembly 730, a RadioFrequency coil assembly 750, a magnetic field gradient coil assemblyalso utilized for the magnetic field source 710 and at least onemagnetic field sensor 720 for tracking the location of a target 780. Inthis embodiment, the homogeneous region of the magnetic field of thestatic magnetic field generating coil assembly is external to the volumeof the magnet assembly 705. Additionally, the magnetic field gradientproduced by the magnetic field gradient coil assembly, and magneticfield generated by the radio frequency assembly are all present at thehomogeneous region. In the embodiment, which is shown in FIG. 7A themagnetic field gradient coil assembly of the magnetic resonance scanneris used also as the magnetic field source for location tracking.Additionally, the magnetic field gradient coil assembly may be usedsolely for the purpose of a magnetic field source if spatial encoding ofthe nuclear spins is not required because the target's location is knownby the location tracking device and the representation of the target maybe created by multiple acquisitions of the magnetic resonance scanner'shomogeneous region. For example, in a preferred embodiment, in which themagnetic resonance scanner is a handheld device and where the homogenousregion of the static magnetic field generating coil assembly is createdoutside of the volume of the entire assembly, spatial encoding is notrequired due to the small size of the homogeneous region. With such ahandheld device, the location tracking device provides the means forbuilding a representation of the target by combining multiple dataacquisitions of the target through movement of either the target or thehandheld magnetic resonance scanner.

FIG. 7B shows a block diagram showing the data and signal connectionsbetween the magnetic resonance scanner and the electromagnetic positionand orientation tracking device including the processor for position andorientation tracking 792 and the magnetic field sensor 720 of FIG. 7A.FIG. 7B is similar to FIG. 1B except that the magnetic field gradientcoil assembly and the magnetic field source are combined into one block710 for the magnetic resonance scanner with position and orientationtracking 700.

The static magnetic field creates a magnetization of the nuclear spins.The processor for magnetic resonance data acquisition 790 maintains ahomogeneous region of the static magnetic field using the staticmagnetic field generating assembly 730 through built in sensors. Suchsensors may measure the current or the magnetic field. The processor 790is also responsible for transmitting radio frequency signals to excitethe nuclear spins of the target and to receive the radio frequencysignals in response of the transmission via the radio frequency coilassembly 750.

To build a representation of the target in three dimensions, the nuclearspins present in the target can first be spatially encoded. Theprocessor 790 can generate time and spatial variation of the staticmagnetic field through the magnetic field gradient generating coilassembly 710 or via collecting multiple data sets from the target in thehomogenous area. By combining the multiple data sets into a new data setbased on the location of the target relative to the homogenous areaprovided by the position and orientation device, a three dimensionalrepresentation can be built.

For example, the gradient coil 710 of the magnetic resonance scannermagnet assembly 705 (of FIG. 7A) generates a linear magnetic fieldgradient across the target 780 (of FIG. 7A) encoding the nuclear spinsin the target 780 according to the magnetic resonance sequence, suchthat, the frequency of the magnetic resonance is linearly dependent onthe position of the nuclear spins relative to the center of the gradientcoil 710. Then a Fourier transform is performed in the processor 790 forfrequency separation. The processor can then retrieve the positiondependent nuclear spin density data from the acquired radio frequencysignal and make the data available to the system processor 795. Inanother embodiment, the magnetic resonance signal is acquired from theentire area of the target 780 present in the homogenous region of thestatic magnetic field generated by the static magnetic field generatingcoil assembly 730.

In this embodiment, it is not possible to position the entire targetwithin the homogeneous region and therefore multiple data acquisitionsmust occur and combined in order to have a complete representation ofthe target. By measuring the target's location relative to thehomogenous region with the data acquisition of the spin density of thetarget, the location may provide a means for combining all dataacquisitions until the representation is satisfactory to a user of thesystem. Then by moving the target 780 relative to the homogenous region,a new data acquisition of the target in the homogenous region can bemeasured along with the new location of the target relative to thehomogenous region and a combined map of the nuclear spin density of thetarget is built.

The system processor 795 is responsible to accept user input 796, fordetermining parameters of the data acquisition, and to present thecombined data for the user output 797. For example, a user may choose adifferent repetition time for the data acquisition sequence. Theprocessor 795 may combine data from data acquisitions archived in dataarchive 798 to help the user to control the data acquisition and analyzethe data presented.

FIG. 8A depicts the generation of a static magnetic field for use in amagnetic resonance scanner in one embodiment of the invention. Toachieve an open, planar magnet configuration for the generation of thestatic magnetic field for use in magnetic resonance scanner, two loopsare employed which are capable of carrying current. A first staticmagnetic field generating coil 831 is driven with electric current. Asecond static magnetic field generating coil 832 is driven with currentwhich is of opposite polarity to the first coil 831. The two coils havecommon axis which passes through the centroid of each coil, and thecoils are substantially parallel to one another. Because the differencein size and separation of the coils, the coils will generate magneticfields with different intensities and spatial gradients.

When both coils, 831, 832 are driven with current simultaneously, ahomogenous region 833 of the generated magnetic fields can form. Throughproper position and sizing of the coils and the magnitude of thecurrents in the coils, a cancellation of the spatial gradient in themagnetic field occurs at a region which is designated as the homogeneousregion. Although the spatial gradients cancel at the homogeneous region,the intensities of the magnetic fields do not. As a result, the combinedresulting intensity in the homogeneous region may be used as the staticmagnetic field of the magnetic resonance scanner. The direction of themagnetic field in the homogeneous region is substantially parallel tothe common coil axis which in FIG. 8A is designated as Z. Thehomogeneous region is formed outside of the volume defined by the firstand second coils 831, 832 as a result of the placement of the coils,size of the coils, and the magnitude and polarity of the current whichflows through the coils.

FIG. 8B shows a graph of the magnetic field intensities of the first andsecond static magnetic field generating coils 835, 836 along the Z axisof FIG. 8A along with the resulting magnetic field intensity 837 withthe homogeneous region 833 being denoted by a substantially constantintensity region.

FIGS. 9A-C describes the generation of the magnetic field gradients usedfor spatial encoding of nuclear spins present at the homogenous regionof the static magnetic field. In one embodiment, the magnetic fieldgradient coil assembly uses four adjacent coplanar coils for both thegeneration of the magnetic field gradient and the magnetic field forposition and orientation tracking. Each of the coils are constructedfrom three segments which are linear. By driving electric current into acombination of coils simultaneously, this configuration enables threedimensional spatial encoding of the spins of the target in thehomogenous area of the static magnetic field.

As shown on FIG. 9A, the first, second, third, and fourth coils 931,932, 933, 934 are driven with electric current simultaneously withidentical polarities. As a result, a magnetic field is generated whichis substantially parallel to the magnetic field present in thehomogenous area of the static magnetic field coil assembly depicted inFIG. 7A, which is in the direction of the Z axis and has a gradient alsoin that direction. The magnetic field created by the current as ittraverses the coils along the dotted line substantially cancels for thepurpose of the magnetic field gradient generation.

As shown in FIG. 9B, when current is carried in in the first coil 931and second coil 933 simultaneously and the current in the first coil 931is opposite in polarity to the current in the second coil 933, amagnetic field is generated which is substantially parallel to thedirection of the magnetic field present in the homogenous area of thestatic magnetic field coil assembly which is along the Z axis, and ithas a magnetic field gradient in the direction from the centroid of thefirst coil 931 to the centroid of the second coil 933 along the X axis,substantially perpendicular to the direction of the static magneticfield present in the homogenous area.

As shown in FIG. 9C, current is carried in the third coil 932 and fourthcoil 934 simultaneously. The current in the third coil 932 is oppositein polarity to the current in the fourth coil 934, as a result, amagnetic field is generated which is substantially parallel to thedirection of the magnetic field present in the homogenous area of thestatic magnetic field coil assembly. This magnetic field has a magneticfield gradient in the direction from the centroid of the third coil 932to the centroid of the fourth coil 934 along the Y axis which issubstantially perpendicular to both the direction of the magnetic fieldgradient generated with the configuration depicted in FIGS. 9A and 9B.

It should be understood by those skilled in the art that the descriptionof the magnetic fields provided with respect to FIGS. 9A, 9B, and 9Cdoes not completely describe the magnetic fields generated by the coils.However, the description provides the magnetic field components whichare necessary for employment in a magnetic resonance scanner. Themagnetic field gradients in combination with the static magnetic fieldprovides a means for the spatial encoding of the nuclear spins in eachof the three directions as described with respect to FIGS. 9A, 9B and 9Cregardless of the incomplete description. The elements of the magneticfields which are not described provide negligible contributions to thespatial encoding of the spins. These elements which are negligible forspatial encoding are not negligible for position and orientationtracking and therefore the model of the magnetic field source shouldaccount for them. For example, for the magnetic field gradientgeneration where the dotted line segments were omitted from thedescription, these elements should be included in the model.

FIGS. 10A and 10B show a Radio Frequency magnet assembly for use in amagnet resonance scanner for the excitation of nuclear spins and for thedetection of a resulting signal as described above. The assemblyprovides for the generation of quadrature magnetic fields, which by thereciprocity principle, allows for detection of magnetic fields with aquadrature sensitivity profile. FIG. 10A is a simplified magnetic fieldgenerating and sensing system. FIG. 10A shows a magnetic field generatedby a pair of adjacent coplanar coils. The pair of adjacent co-planarcoils is electrically connected together with opposing polarity and hasa direction of sensitivity parallel to the plane of the coils above thecentroid of the combined structure of the coil pair. The direction ofthe sensitivity is also parallel to the direction from the centroid ofthe first coil to the centroid of the second coil as shown between coilpairs (1040 and 1020).

FIG. 10B shows two pairs of adjacent co-planar coils (1010 1030, 10401020) having sensitivity direction perpendicular to each other. This isthe preferred embodiment of the radio frequency coil assembly, sincethis configuration enables simultaneous transmission and detection orquadrature detection of radio frequency magnetic fields. The detectedsignals from each of the coil pairs are provided to the processor of themagnetic resonance scanner and becomes the acquired data.

The disclosed apparatus and method may be implemented in combination ofhardware and software. Representations for hardware are passiveelectronic components for example resistors capacitors, inductors, coilsand active electronic components such as a transistor or more complexintegrated circuits of analog and digital nature, such as operationalamplifiers or logic circuits. Implementation of the method requiresexecution of computer instructions or operations. Fixed orre-programmable devices may be employed such as different types of ROM(Read Only Memory), RAM (Random Access Memory), or FPGAs (FieldProgrammable Gate Arrays), CPLDs (Complex Programmable Logic Devices) ormicroprocessors. These offer alternatives for implementation, forexample trading speed of execution for implementation cost. Suchimplementation may include a series of computer instructions fixedeither on a tangible medium, such as a computer readable media (e.g., adiskette, CD-ROM, ROM, or fixed disk) or transmittable to a computersystem, via a communications adapter connected to a network over amedium, such as Ethernet or modem or other interface device. Medium maybe either a tangible medium (e.g., wire or optical communication lines)or a medium implemented with wireless techniques (e.g., depend on thefrequency, RF, microwave or light or other transmission techniques). Theseries of computer instructions embodies all or part of thefunctionality previously described herein with respect to the system.Those skilled in the art should appreciate that such computerinstructions can be written in a number of programming languages for usewith many computer architectures or operating systems. Furthermore, suchinstructions may be stored in any memory device, such as semiconductor,magnetic, optical or other memory devices, and may be transmitted usingany communications technology, such as optical, infrared, microwave, orother transmission technologies. It is expected that such a computerprogram product may be distributed as a removable media withaccompanying printed or electronic documentation (e.g., shrink wrappedsoftware), preloaded with a computer system (e.g., on system ROM orfixed disk), or distributed from a server or electronic bulletin boardover the network (e.g., the Internet or World Wide Web).

Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention. These and other obvious modifications are intended to becovered by the appended claims.

1. A system comprising: a magnetic field source generating a spatiallydistinct magnetic field from a source signal; a magnetic field sensorsensing a sensed magnetic field; and a processor receiving a sensorsignal from the magnetic field sensor, wherein the processor calculatesa position and orientation of the magnetic field sensor relative to themagnetic field source, from: a) a magnetic field source model; b) arepresentation of the source signal from the magnetic field source; c) amagnetic field sensing model; d) a representation of the sensor signalfrom the magnetic field sensor; wherein the magnetic field source modelincludes a representation of tangible elements of the magnetic fieldsource and a representation of tangible magnetically coupled elementswhich contribute to the sensed magnetic field.
 2. The system accordingto claim 1, wherein the processor provides a signal representative ofthe position and orientation of the magnetic field sensor to an output.3. The system according to claim 1, wherein the magnetic field sourceand the magnetic field sensor include a number of tangible elements suchthat the number of possible independent measurements exceeds the minimumnumber required to determine the position and orientation of themagnetic field sensor; and the processor: collects at least the minimumnumber of independent measurements to calculate a value for the tangiblemagnetically coupled elements of the magnetic field source model and themagnetic field sensing model.
 4. The system according to claim 1,wherein a second sensor provides a number of tangible elements such thatthe number of possible independent measurements exceeds the minimumnumber required to determine the position and orientation of the firstand second magnetic field sensors.
 5. The system according to claim 1,wherein the processor uses a line segment model for the magnetic fieldsource model.
 6. The system according to claim 1, wherein at least onemagnetic field sensor is attached to at least one moveable object andthe processor calculates the position and orientation of the at leastone moveable object relative to the magnetic field source.
 7. The systemaccording to claim 6, wherein a magnetic resonance scanner is includedfor scanning a target.
 8. The system according to claim 7, wherein themagnetic field source is coupled to a magnet assembly of the magneticresonance scanner.
 9. The system according to claim 8, wherein themagnetic resonance scanner further includes a gradient magnetic fieldgenerating element for scanning the target and the gradient magneticfield generating element also utilized as the magnetic field source. 10.The system according to claim 8, wherein the magnetic resonance scanneroutputs a scan data set representative of an area of the target for theprocessor and the processor combines the scan data set with the positionand orientation of the moveable object creating an output.
 11. Thesystem according to claim 10, wherein the output is displayed on adisplay device.
 12. The system according to claim 10, furthercomprising: a third magnetic field sensor is attached to the magnetassembly of the magnetic resonance scanner, instead of the magneticfield source.
 13. The system according to claim 12, further comprising:a fourth magnetic field sensor coupled to the target, wherein theposition and orientation tracking device is capable of tracking theposition and orientation of the target relative to the magnet assemblyof the magnetic resonance scanner.
 14. A method for determining theposition and orientation of a magnetic field sensor relative to amagnetic field source, comprising: generating a spatially distinctmagnetic field from a source signal with a magnetic field source;providing a sensor signal proportional to a sensed magnetic field from asensor to a processor; and calculating a position and orientation of themagnetic field sensor relative to the magnetic field source with theprocessor, from: a magnetic field source model; the source signal; amagnetic field sensing model; the sensor signal; and wherein themagnetic field source model includes a representation of tangibleelements of the magnetic field source and representations of a tangiblemagnetically coupled elements which contribute to the sensed magneticfield.
 15. The method according to claim 14, further comprising:providing a signal representative of the position and orientation of themagnetic field sensor to an output.
 16. The method according to claim14, wherein the processor collects independent measurements in excess ofthe minimum required to compute the position and orientation of themagnetic field sensor relative to the magnetic field source, and usesthe measurements also to redefine the tangible magnetically coupledelements of the magnetic field source model and the magnetic fieldsensing model to achieve better accuracy.
 17. The method according toclaim 16, wherein the magnetic field source and the magnetic fieldsensor include a number of tangible elements such that the number ofpossible independent measurements exceeds the minimum number required todetermine the position and orientation of the magnetic field sensor. 18.The method according to claim 16, wherein a second sensor provides anumber of tangible elements such that the number of possible independentmeasurements exceeds the minimum number required to determine theposition and orientation of the first and second magnetic field sensors.